Learning Objectives
By the end of this activity, students will be able to:
Explain the relationship between electricity and magnetism.
Describe how a coil carrying current interacts with a magnetic field to create motion.
Observe how electric motors convert electrical energy into mechanical energy.
Relate their experiment to real-world nondestructive testing applications.
Key Concepts
An electric motor consists of two parts, the stator and the rotor. In a motor, the stator is the part that remains motionless, and the rotor is the part that moves.
The basic principle for all working motors is magnetic attraction and repulsion. Since a magnet no longer moves once it has attracted, a motor needs some way to manipulate the magnetic fields so that the magnets attract and repel continuously.
One way to do this is to have the current changing directions. Since alternating current (AC) electricity alternates, it naturally reverses magnetic fields with each change.
Current in the rotor coils causes the rotor to turn in a magnetic field. Turning the rotor in a magnetic field causes a current to flow in the coils.
Except for friction losses, power in equals power out.
When acting as a generator, attaching an electrical load to the output allows a current to flow. The current flow establishes a magnetic field producing forces opposing the mechanical agent turning the rotor.
When acting as a motor, the motor running at the designed speed has a rapidly changing magnetic flux inducing back electromotive force (EMF), so the rotor coil passes very little current.
Attaching a mechanical load to the output slows the motor reducing the changing flux, thus reducing the back EMF, and the motor passes more current. This is why electric motors pull a lot of currents when they first start up and are doing a lot of work getting things up to speed.
